In many technical applications, especially in connection with radial presses and collet chucks, three or more radial forces may be acting centrally, i.e. toward a central axis of measurement. In the case of a radial press, for example, the number of radial forces occurring may be determined by the number of press jaws present. A need exists to sense the said radial forces in their entirety in the sense of a spatially integrating measurement and to derive therefrom an effective force that represents the superposition of the acting radial forces.
DE 68915875 T2 discloses a device with which a force of a fluid medium acting radially on several ribs of a ribbed cage is converted into an amplified axially acting force of a thrust rod coupled with the ribs. To convert the radial force into the axial force, the ribs can be biased by means of a spring, the spring tension of which relaxes in the course of time. The same is true for the ribs, when these are made of elastic material. By means of the axial force of the thrust rod, a switch can be actuated. But DE 68915875 T2 does not yet provide any indication of how to use the device for measuring forces. It would not even be very suitable for doing so, because the compliance of the many elastic elements would act detrimentally on the measurement accuracy. Furthermore, the ribs according to the invention are very weak and cannot withstand any large forces. Especially if the ribs were to be fastened so to swivel on one another, as is also an alternative objective, the device would not be capable of withstanding and measuring high forces, as occur, for example, in radial presses or during the use of collet chucks.
DE 4025353 A1 discloses two substantially cylindrical sensors for measuring forces acting perpendicularly relative to their respective shell surfaces. As an example, it is provided that the sensor will be driven into a bore, in order to measure continuous radial force variations therein. According to one embodiment, the sensor is equipped for this purpose with a tubular piezoelectric double crystal, which responds to radial forces. Such a solution is very expensive. According to a further embodiment, a sensor body filled with oil is provided. When radial forces act on the sensor body, they cause, in the oil present therein, a pressure change, which is transmitted by a piezoresistive sensor. An electromechanical transducer arrangement is necessary to measure the force due to the described two sensors, since direct measurement with purely mechanical means is not possible. Furthermore, the sensors are suitable more for long-time dynamic force changes and less for rapidly changing radial forces, such as occur in a radial press or a collet chuck.
U.S. Pat. No. 4,259,863 A discloses an extremely complex device for measuring forces and bending moments in and around the x, y and z axes by means of a plurality of sensors and springs. However, this device has only very limited suitability for sensing radial forces.
DE 1059687 B teaches an instrument for measuring the mean tension of several wires clamped parallel to one another. Therein that force which is necessary to spread the wires is determined as a measure of the mean tension in the wires clamped parallel to one another. For this purpose, the instrument is equipped with an elongated expander member, which extends along the axis and is elastically deformable in axial direction, several knee levers, which are disposed around the expander member, can be subjected to force by the wires and act on the expander member, and means for measuring the change in length of the expander member. This change in length of the expander element is determined by the vibrational behavior of a string, which is formed by a wire clamped in the expander element along the axis and which is fixed to the ends of both spreader members. The frequency of the string—which varies with the length expansion—is sensed by means of electromagnets, which are attached to the expander element itself and also provide vibrational excitation of the string.
DE 1020050505787 A1 discloses a joining element, which is suitable for measuring a vertical force. It has a component that can be subjected to a vertical force and is disposed between two spring-biased, conical plates guided displaceably on a horizontal axis. The degree of displacement of the two plates relative to one another on the axis represents a measure of the axial force acting on the component.
KR 1020090120326 A relates to the determination of tensions in a pipe during the axial expansion thereof and/or of properties of the material of an axially expanded pipe.
In this respect, further technological background of the invention is also represented—albeit without disclosure of any expander element arrangement of the type mentioned in the introduction—by DE 10051010 A1 and EP 0419129 A1. DE 10051010 A1 discloses a force-measuring tool in which two levers are adapted to the press geometry of a four-mandrel press head and act on a pressure sensor during compression. EP 0419129 A1 discloses a press with force-measuring devices, such as strain gauges, disposed in the press.
None of the cited devices make it satisfactorily possible to achieve an accurate and reliable integrating measurement in the foregoing sense with simple means, especially in radial presses or collet chucks.
In view of the problems, restrictions and disadvantages of the devices known from the prior art as described in the foregoing, it is the object of the invention to provide a simple force-measuring device, with which an improved, integrating measurement of radial forces is possible, especially in radial presses or collet chucks.